1. Field of the Invention
The present invention relates to a superconducting magnet.
2. Description of the Related Art
There have been known two methods of operating a superconducting magnet. One is a method of constantly applying an electric current from a power source to the superconducting magnet. The other is a method called a persistent current mode, in which an electric current persistently flows in a superconducting loop even without current supply from a power source, making use of a superconducting phenomenon of zero electrical resistance.
FIG. 6 is an electrical diagram showing a configuration of a general superconducting magnet provided with a persistent current mode.
In FIG. 6, a superconducting magnet 99 and a persistent current switch 100 are connected in parallel to a power source 101. Wiring for connecting the superconducting magnet 99 and the persistent current switch 100 are made of a superconducting material.
The persistent current switch 100 is turned OFF, if an electrical current is applied to the superconducting magnet 99. The persistent current switch 100 is designed to have an electrical resistance of ten and several ohms, when the persistent current switch 100 is turned OFF. At this time, an electrical resistance of the superconducting magnet 99 is zero. This allows an electric current supplied from the power source 101 to flow to the superconducting magnet 99, which has less electrical resistance. Thus, a current value applied to the superconducting magnet 99 can be manipulated by way of the external power source 101.
After the electric current supplied to the superconducting magnet 99 reaches a designed value, the persistent current switch 100 is turned ON. The persistent current switch 100 is in a superconducting state with zero electrical resistance, when the persistent current switch 100 is turned ON. At this time, the superconducting magnet 99 and the persistent current switch 100 turn superconductive and form a closed circuit or a superconducting loop. An electric current in the superconducting loop flows persistently without attenuation. This eliminates a need of further supplying an electric current from the power source 101, thus enabling the power source 101 to be cut off from the superconducting loop. The superconducting magnet 99 can be then operated in the persistent current mode.
To achieve a magnet having a strong magnetic field, a current as high as multi-hundred amperes is necessary. To keep on supplying an electric current from an external power source, a good amount of running cost is necessary. Meanwhile, if a superconducting magnet is operated in the persistent current mode, running cost is not necessary other than a cost during initial phases of its start-up, because, in principle, only one excitation is enough to persistently operate the superconducting magnet in the persistent current mode. Thus, the persistent current switch is widely used in the superconducting magnet.
Japanese Published Patent Application No. 2003-037303 indicates the following characteristics required for a persistent current switch: 1) its electrical resistance when turned ON is zero or within a range in which magnetic field attenuation is acceptable; 2) its electrical resistance is sufficiently large when turned OFF; 3) it allows an electric current to be supplied to a coil; and 4) its superconducting state is stable.
A superconducting material has no electrical resistance in the superconducting state but does have in a normal conducting state. That is, a persistent current switch can be turned ON or OFF by switching between the superconducting state and the normal conducting state from outside.
Though a persistent current switch can be made of various superconducting materials, niobium-titanium (NbTi) is typically used. NbTi has a critical temperature for superconductors of about 9K and enters a normal conducting state at about 9K or higher. In operation, the persistent current switch is immersed in liquid helium whose boiling point is 4.2K and is thus cooled to 4.2K. To turn OFF, the persistent current switch is required to be heated to the critical temperature thereof for superconductors (in case of NbTi, about 9K) or higher. The persistent current switch is typically turned OFF by heating a noninductive winding part thereof to the critical temperature for superconductors using a heater.
To turn the persistent current switch from OFF to ON, the heater housed in the persistent current switch is just turned off. The persistent current switch is then allowed to be cooled by a surrounding liquid helium until the persistent current switch has the same temperature as that of the surrounding liquid helium after a certain period of time. Japanese Published Patent Application No. H09-298320 proposes a configuration of a persistent current switch in which a heater is arranged for promoting a rise in temperature, and evaporated helium gas is stored around the persistent current switch for enhancing heating efficiency.
However, the persistent current switch using NbTi has a problem. The NbTi-using persistent current switch has the critical temperature for superconductors of as low as about 9K, of which temperature difference from the surrounding liquid helium (boiling point: 4.2K) is small. Even a slight rise in temperature owing to very little heat generation causes the persistent current switch to transit from the superconducting state to the normal conducting state. In general, the lower a temperature of an object becomes, the smaller a specific heat of the object becomes. In a system such as a persistent current switch, in which a large current flows, a localized heat generation can push a portion of the persistent current switch into the normal conducting state, and an electrical resistance produced in the portion generates more heat, to thereby end up with a quench.
The quench not only causes the persistent current switch to stop operation but may result in fatal damage thereto, in particular, to a high-field magnet in which a huge amount of energy is stored.
Japanese Published Patent Application No. 2003-037303 discloses that usage of a material having a high critical temperature for superconductors is effective for enhancing stability of a superconducting material and proposes a persistent current switch using MgB2 (critical temperature for superconductors: 39K) and having more stability.
As described above, the persistent current switch can have a stable superconducting state by using a superconducting material having a high critical temperature for superconductors such as, for example, MgB2 (critical temperature for superconductors: 39K). There is a problem, however, that a higher critical temperature for superconductors increases a heating amount to be provided by a heater. The heater is used for raising a temperature of the persistent current switch from that of the liquid helium in which the persistent current switch is cooled, to the critical temperature or higher. Another problem is that an increase in the heating amount also increases evaporation of the liquid helium.
FIG. 7 is a cross sectional view showing a configuration of a persistent current switch according to a related art.
In FIG. 7, a persistent current switch includes as principal components a winding part 20 and an extracting part 21. The winding part 20 includes a wire 10 wound around a bobbin 12. The wire 10 used herein is made of a superconducting material.
The extracting part 21 includes: the wire 10 extending from the winding part 20; a NbTi wire 11 connected to a superconducting magnet (not shown); and a superconducting connection 15 for connecting the wire 10 and the NbTi wire 11, all of which form a superconducting loop.
The wire 10 is required to have a certain length for obtaining an electrical resistance in the normal superconducting state. The wire 10 is herein wound around the bobbin 12 for making the winding part 20 compact. If the wire 10 is just simply wound therearound, however, an inductive magnetic field having a main axis in an axial direction of the bobbin 12 will be generated when an electric current flows in the wire 10. To avoid this, the wire 10 is noninductively wound. That is, the wire 10 is first bent in half, and then, the doubled wire 10 is wound around the bobbin 12 as it is.
A winding-heating heater 5 is fixed so as to be thermally in contact with the wire 10 noninductively wound around the bobbin 12. An electrical insulator (not shown) is inserted between the winding-heating heater 5 and the wire 10, thereby preventing a short circuit.
The wire 10 used herein is made of magnesium diboride (MgB2). MgB2 is characterized by its high superconducting critical temperature as high as 39K and is a relatively new superconducting material reported in Nature 410, 63-64 (2001). Niobium tin (Nb3Sn) is a well-known metallic superconducting material, but its superconducting critical temperature is about 23K. The superconducting critical temperature of MgB2 is about 15K higher than Nb3Sn. The higher the superconducting critical temperature of a superconducting material is, compared to the temperature of the liquid helium (4.2K) as a cryogen, the more the superconducting material has stability.
The persistent current switch according to the related art as described above was manufactured and was inserted into a vessel filled with liquid helium. A heat quantity necessary for turning ON or OFF of the persistent current switch was measured. The switch according to the related art required a heating amount of about 60 W.
In the meantime, an evaporative latent heat of the liquid helium is 20.7 kJ/kg. A density thereof is 124.9 kg/m3. Based on these values, a consumption of the liquid helium, which will evaporate, is measured to be 80 liters or more per hour. The liquid helium is a highly-priced cryogen, and an increase in the consumption of the liquid helium directly leads to an increase in operating cost.
The persistent current switch consumes a large quantity of liquid helium, because, in regulating a magnetic field flowing in a superconducting coil, the switch is required to keep an OFF state thereof, that is, to maintain an ON state of a heater.
In this regard, a persistent current switch using conventional NbTi does not require a large heating amount inputted by a heater because the critical temperature for superconductors of NbTi is low. A heater of about 5 W is sufficient to turn the switch ON or OFF.
As described above, there is a problem that a persistent current switch using MgB2 or any other superconducting material having a high critical temperature for superconductors requires cost of the liquid helium ten times more or higher than that using the conventional superconducting material.
To achieve a practical use of a persistent current switch using the material having a high critical temperature for superconductors such as MgB2, there is a need for a configuration in which heat load of a heater necessary to turn the persistent current switch ON or OFF is lowered.
To solve the above problems, the present invention has been made in an attempt to provide a superconducting magnet provided with a persistent current switch capable of lowering a heating amount which is necessary for a heater to turn the persistent current switch OFF, even if the persistent current switch is made of a superconducting material having a high critical temperature for superconductors.